Sensor element and gas sensor

ABSTRACT

A sensor element includes element main body including side surfaces, a detection unit, connector electrodes disposed on the rear end-side part of the side surfaces, a porous layer that covers at least front end-side part of the side surface, the porous layer having a porosity of 10% or more, and a water-penetration reduction portion. The water-penetration reduction portion is disposed on the side surface so as to divide the porous layer or to be located closer to the rear end than the porous layer. The length L of the water-penetration reduction portion is 0.5 mm or more. The water-penetration reduction portion includes, among a dense layer covering the side surface and having a porosity of less than 10% and a gap region in which the porous layer is absent, at least the dense layer. The water-penetration reduction portion reduces the capillarity of water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/658,340, filed Oct. 21, 2019, which is a continuation applicationof PCT/JP2019/001849, filed on Jan. 22, 2019, which claims the benefitof priority of Japanese Patent Application No. 2018-019444, filed onFeb. 6, 2018, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a sensor element and a gas sensor.

2. Description of the Related Art

Sensor elements that detect the specific gas concentration, such as NOx,in the measurement-object gas, such as an automotive exhaust gas, areknown (e.g., PTL 1). The sensor element disclosed in PTL 1 includes amultilayer body that includes oxygen ion-conducting solid electrolytelayers stacked on top of one another. This sensor element also includesan outer pump electrode, a lead wire for the outer pump electrode, aconnector electrode, and a porous protection layer, which are stacked onand above the upper surface of the multilayer body. The outer pumpelectrode, the lead wire for the outer pump electrode, and the connectorelectrode are connected to one another in this order and are inelectrical conduction with one another. The connector electrode iselectrically connected to the outside. The porous protection layercovers and protects the outer pump electrode and the lead wire for theouter pump electrode.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No.2016-014659

SUMMARY OF THE INVENTION

When a porous layer similar to the porous protection layer described inPTL 1 is present on the surface of the sensor element, the moisturecontained in an exhaust gas may move inside the porous layer bycapillarity. As a result, the moisture may reach the connectorelectrode. In such a case, water and the components dissolved in water,such as sulfuric acid, cause rusting and corrosion of the connectorelectrode and a short circuit between the connector electrodes.

The present invention was made in order to address the above issues. Anobject of the present invention is to prevent the moisture from passingthrough the porous layer and reaching the connector electrodes.

Solution to Problem

The present invention employs the following structures in order toachieve the object.

A sensor element of the present invention includes:

a long-length element main body including front and rear ends and one ormore side surfaces, the front and rear ends being ends of the elementmain body in a longitudinal direction of the element main body, the oneor more side surfaces being surfaces extending in the longitudinaldirection;

a detection unit including a plurality of electrodes disposed in thefront end-side part of the element main body, the detecting unitdetecting the specific gas concentration in the measurement-object gas;

one or more connector electrodes disposed on the rear end-side part ofany of the one or more side surfaces, the one or more connectorelectrodes being in electrical conduction with the outside;

a porous layer that covers at least the front end-side part of the sidesurface on which the one or more connector electrodes are disposed, theporous layer having a porosity of 10% or more; and

a water-penetration reduction portion disposed on the side surface so asto divide the porous layer in the longitudinal direction or to belocated closer to the rear end than the porous layer, thewater-penetration reduction portion being located closer to the frontend than the one or more connector electrodes, the length L of thewater-penetration reduction portion in the longitudinal direction being0.5 mm or more, the water-penetration reduction portion including, amonga dense layer covering the side surface and having a porosity of lessthan 10% and a gap region in which the porous layer is absent, the gapregion being arranged adjacent to the dense layer, at least the denselayer, the water-penetration reduction portion reducing the capillarityof water in the longitudinal direction.

In the above-described sensor element, the connector electrodes aredisposed on a rear end-side part of any of the one or more side surfacesof the element main body, and the porous layer is arranged to cover atleast the front end-side part of the side surface. Furthermore, thesensor element includes the water-penetration reduction portion disposedon the side surface so as to divide the porous layer in the longitudinaldirection or to be located closer to the rear end than the porous layer.The water-penetration reduction portion is located closer to the frontend than the connector electrode. Therefore, even when the frontend-part of the element main body, in which a plurality of electrodesconstituting the detection unit are present, is exposed to themeasurement-object gas and the moisture contained in themeasurement-object gas moves inside the porous layer toward the rear endof the element main body by capillarity, the moisture reaches thewater-penetration reduction portion before reaching the connectorelectrodes. In the water-penetration reduction portion, which includes,of the dense layer and the gap region, at least the dense layer, thecapillarity of water in the longitudinal direction of the element mainbody is not likely to occur, unlike in the porous layer. In addition,since the length L of the water-penetration reduction portion in thelongitudinal direction is 0.5 mm or more, the likelihood of the moisturepassing through the water-penetration reduction portion can be reducedto a sufficient degree. By the above mechanisms, the water-penetrationreduction portion reduces the likelihood of the moisture passing throughthe water-penetration reduction portion and reaching the connectorelectrodes. Accordingly, the above sensor element is capable of reducingthe likelihood of the moisture passing through the porous layer andreaching the connector electrodes. In the above case, the porosity ofthe dense layer may be 8% or less or may be 5% or less.

In the sensor element according to the present invention, the length Leof the dense layer in the longitudinal direction may be 0.5 mm or more.In such a case, the likelihood of the moisture passing through thewater-penetration reduction portion in the longitudinal direction can bereduced to a sufficient degree by using only the dense layer of thewater-penetration reduction portion.

In the sensor element according to the present invention, the length Leof the dense layer in the longitudinal direction may be 20 mm or less.In such a case, for example, in the case where the element main body andthe dense layer are prepared by baking an unbaked element main body andan unbaked dense layer, the warpage of the sensor element due to thedifference in shrinkage ratio during baking between the unbaked elementmain body and the unbaked dense layer can be reduced.

In the sensor element according to the present invention, the length Leof the dense layer in the longitudinal direction may be 30% or less ofthe length of the element main body in the longitudinal direction. Insuch a case, for example, in the case where the element main body andthe dense layer are prepared by baking an unbaked element main body andan unbaked dense layer, the warpage of the sensor element due to thedifference in shrinkage ratio during baking between the element mainbody and the dense layer can be reduced.

In the sensor element according to the present invention, the length Lgof the gap region in the longitudinal direction may be 1 mm or less. Insuch a case, since the length Lg of the gap region is relatively small,the area of a part of the side surface of the element main body which isexposed to the outside (the part that is not covered with any of theporous layer and the dense layer) can be reduced.

In the sensor element according to the present invention, thewater-penetration reduction portion does not necessarily include the gapregion. In other words, the length Lg of the gap region of thewater-penetration reduction portion in the longitudinal direction may be0 mm. In such a case, the area of a part of the side surface of theelement main body which is exposed to the outside (the part that is notcovered with any of the porous layer and the dense layer) can be furtherreduced.

The sensor element according to the present invention may furtherinclude an outer lead portion disposed on the side surface on which theone or more connector electrodes are disposed, the outer lead portionproviding conduction between any of the electrodes and the one or moreconnector electrodes. The porous layer may cover at least a part of theouter lead portion. This enables at least a part of the outer leadportion to be protected with the porous layer. In the case where theouter lead portion is protected with the porous layer, the porous layeris likely to be disposed at a position close to the one or moreconnector electrodes and, therefore, it is meaningful to apply thepresent invention to such a sensor element.

In the above case, the porous layer may cover the entirety of the outerlead portion. Alternatively, the porous layer may cover the entirety ofthe part of the outer lead portion on which the water-penetrationreduction portion is not present. The sensor element according to thepresent invention may include an outer electrode that is one of theelectrodes included in the detection unit, the outer electrode being inconduction with the connector electrodes via the outer lead portion anddisposed on the side surface on which the connector electrodes aredisposed. In such a case, the porous layer may cover the outerelectrode.

In the sensor element according to the present invention, the porouslayer may cover at least a region of the side surface on which the oneor more connector electrodes are disposed, the region extending from thefront end of the side surface to the front end-side edge of the one ormore connector electrodes, the region excluding a region in which thewater-penetration reduction portion is present, and thewater-penetration reduction portion may be disposed on the side surfaceso as to divide the porous layer in the longitudinal direction.

In the sensor element according to the present invention, the elementmain body may have a rectangular cuboid shape and four side surfacesthat are surfaces extending in the longitudinal direction, one or moreconnector electrodes may be disposed on each of first and second sidesurfaces of the four side surfaces, the first and second side surfacesfacing each other, the porous layer may cover each of the first andsecond side surfaces, and the water-penetration reduction portion may bedisposed on each of the first and second side surfaces. In the abovecase, the element main body may be a multilayer body constituted by aplurality of layers stacked on top of one another, and the first andsecond side surfaces may be the upper and lower surfaces of the elementmain body when the direction in which the layers are stacked isconsidered the top-to-bottom direction.

The gas sensor according to the present invention includes the sensorelement according to any one of the above-described aspects. Therefore,the gas sensor has the same advantageous effects as the above-describedsensor element according to the present invention. That is, for example,the gas sensor is capable of reducing the likelihood of the moisturepassing through the porous layer and reaching the connector electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a gas sensor 10attached to a pipe 58.

FIG. 2 is a perspective view of a sensor element 20.

FIG. 3 is a cross-sectional view taken along the line A-A in FIG. 2 .

FIG. 4 is a top view of a sensor element 20.

FIG. 5 is a bottom view of a sensor element 20.

FIG. 6 is a top view of a modification example of a sensor element 20.

FIG. 7 is a graph illustrating changes in penetration distance with timewhich were measured in liquid penetration tests conducted inExperimental examples 1 and 16.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below with referenceto the attached drawings. FIG. 1 is a longitudinal cross-sectional viewof a gas sensor 10 according to an embodiment of the present inventionwhich is attached to a pipe 58. FIG. 2 is a perspective view of a sensorelement 20 viewed from the upper right front. FIG. 3 is across-sectional view taken along the line A-A in FIG. 2 . FIG. 4 is atop view of the sensor element 20. FIG. 5 is a bottom view of the sensorelement 20. In this embodiment, as illustrated in FIGS. 2 and 3 , thelongitudinal direction of the element main body 60 included in thesensor element 20 is referred to as “front-to-rear direction” (lengthdirection), the direction in which the layers constituting the elementmain body 60 are stacked (thickness direction) is referred to as“top-to-bottom direction”, and a direction perpendicular to thefront-to-rear direction and the top-to-bottom direction is referred toas “left-to-right direction” (width direction).

As illustrated in FIG. 1 , the gas sensor 10 includes an assembly 15, anut 47, an external cylinder 48, a connector 50, lead wires 55, and arubber stopper 57. The assembly 15 includes a sensor element 20, aprotective cover 30, and an element-sealing member 40. The gas sensor 10is attached to a pipe 58, such as an automotive exhaust gas pipe, andused for measuring the specific gas concentration, such as NOx or O₂,(particular gas concentration) in the exhaust gas, which is the gas tobe analyzed. In this embodiment, the gas sensor 10 is a gas sensor thatmeasures NOx concentration as a particular gas concentration. Among theends (front and rear ends) of the sensor element 20 in the longitudinaldirection, the front end-side part of the sensor element 20 is exposedto the measurement-object gas.

The protective cover 30 includes, as illustrated in FIG. 1 , a hollowcylindrical inner protective cover 31 with a bottom which covers thefront end-part of the sensor element 20 and a hollow cylindrical outerprotective cover 32 with a bottom which covers the inner protectivecover 31. Each of the inner and outer protective covers 31 and 32 has aplurality of holes formed therein, through which the measurement-objectgas is passed. The space surrounded by the inner protective cover 31serves as an element chamber 33. A fifth surface 60 e (front end-sidesurface) of the sensor element 20 is located inside the element chamber33.

The element-sealing member 40 is a member with which the sensor element20 is sealed and fixed. The element-sealing member 40 includes acylindrical body 41 including a main fitting 42 and an inner cylinder43, insulators 44 a to 44 c, compacts 45 a and 45 b, and a metal ring46. The sensor element 20 is located on the central axis of theelement-sealing member 40 and penetrates the element-sealing member 40in the front-rear direction.

The main fitting 42 is a hollow cylindrical member made of a metal. Thefront-side part of the main fitting 42 is a thick-wall portion 42 ahaving a smaller inside diameter than the rear-side part of the mainfitting 42. The protective cover 30 is attached to a part of the mainfitting 42 which is on the same side as the front end-side of the sensorelement 20 (front-side part of the main fitting 42). The rear end of themain fitting 42 is welded to a flange portion 43 a of the inner cylinder43. A part of the inner peripheral surface of the thick-wall portion 42a serves as a bottom surface 42 b, which is a stepped surface. Thebottom surface 42 b holds the insulator 44 a such that the insulator 44a does not protrude forward.

The inner cylinder 43 is a hollow cylindrical member made of a metal andincludes the flange portion 43 a formed at the front end of the innercylinder 43. The inner cylinder 43 and the main fitting 42 are coaxiallyfixed to each other by welding. The inner cylinder 43 includes adiameter reduction portion 43 c that presses the compact 45 b toward thecentral axis of the inner cylinder 43 and a diameter reduction portion43 d that presses the insulators 44 a to 44 c and the compacts 45 a and45 b in the downward direction in FIG. 1 with the metal ring 46interposed therebetween, the diameter reduction portions 43 c and 43 dbeing formed in the inner cylinder 43.

The insulators 44 a to 44 c and the compacts 45 a and 45 b areinterposed between the inner peripheral surface of the cylindrical body41 and the sensor element 20. The insulators 44 a to 44 c serve as asupport for the compacts 45 a and 45 b. Examples of the material for theinsulators 44 a to 44 c include ceramics, such as alumina, steatite,zirconia, spinel, cordierite, and mullite, and glass. The compacts 45 aand 45 b are formed by, for example, molding a powder and serve as asealing medium. Examples of the material for the compacts 45 a and 45 binclude talc and ceramic powders, such as an alumina powder and boronnitride. The compacts 45 a and 45 b may include at least one of theabove materials. The compact 45 a is filled between the insulators 44 aand 44 b and pressed by the insulators 44 a and 44 b as a result of both(front and rear) ends of the compact 45 a in the axial direction beingsandwiched therebetween. The compact 45 b is filled between theinsulators 44 b and 44 c and pressed by the insulators 44 b and 44 c asa result of both (front and rear) ends of the compact 45 b in the axialdirection being sandwiched therebetween. The insulators 44 a to 44 c andthe compacts 45 a and 45 b are sandwiched between the diameter reductionportion 43 d and the metal ring 46, and the bottom surface 42 b of thethick-wall portion 42 a of the main fitting 42 and thereby pressed inthe front-to-rear direction. As a result of the compacts 45 a and 45 bbeing compressed between the cylindrical body 41 and the sensor element20 by the pressing force applied by the diameter reduction portions 43 cand 43 d, the compacts 45 a and 45 b seal the communication between theelement chamber 33 formed inside the protective cover 30 and a space 49created inside the external cylinder 48 and fix the sensor element 20.

The nut 47 is fixed to the outer surface of the main fitting 42coaxially with the main fitting 42. The nut 47 includes a male threadportion formed in the outer peripheral surface of the nut 47. The malethread portion is inserted into a fixing member 59, which is welded tothe pipe 58 and includes a female thread portion formed in the innerperipheral surface of the fixing member 59. This enables the gas sensor10 to be fixed to the pipe 58 while the front end-side part of thesensor element 20 of the gas sensor 10 and the protective cover 30 ofthe gas sensor 10 are protruded toward the inside of the pipe 58.

The external cylinder 48 is a hollow cylindrical member made of a metaland covers the inner cylinder 43, the rear end-side part of the sensorelement 20, and the connector 50. The upper part of the main fitting 42is inserted into the external cylinder 48. The lower end of the externalcylinder 48 is welded to the main fitting 42. A plurality of the leadwires 55, which are connected to the connector 50, are drawn from theupper end of the external cylinder 48 to the outside. The connector 50is in contact with upper and lower connector electrodes 71 and 72disposed on the rear end-side parts of the surfaces of the sensorelement 20 and electrically connected to the sensor element 20. The leadwires 55 are in electrical conduction with electrodes 64 to 68 and aheater 69 disposed inside the sensor element 20 via the connector 50.The gap between the external cylinder 48 and the lead wires 55 is sealedwith the rubber stopper 57. The space 49 inside the external cylinder 48is filled with a reference gas. A sixth surface 60 f (rear end-sidesurface) of the sensor element 20 is located inside the space 49.

The sensor element 20 includes an element main body 60, a detection unit63, a heater 69, an upper connector electrode 71, a lower connectorelectrode 72, a porous layer 80, and a water-penetration reductionportion 90 as illustrated in FIGS. 2 to 5 . The element main body 60includes a multilayer body constituted by a plurality of (6 layers inFIG. 3 ) oxygen ion-conducting solid-electrolyte layers composed ofzirconia (ZrO₂) or the like which are stacked on top of one another. Theelement main body 60 has a long-length, rectangular cuboid shape, andthe longitudinal direction of the element main body 60 is parallel tothe front-to-rear direction. The element main body 60 has first to sixthsurfaces 60 a to 60 f, which are the upper, lower, left, right, front,and rear outer surfaces of the element main body 60. The first to fourthsurfaces 60 a to 60 d are surfaces that extend in the longitudinaldirection of the element main body 60 and correspond to the sidesurfaces of the element main body 60. The fifth surface 60 e is thefront end-side surface of the element main body 60. The sixth surface 60f is the rear end-side surface of the element main body 60. Thedimensions of the element main body 60 may be, for example, 25 mm ormore and 100 mm or less long, 2 mm or more and 10 mm or less wide, and0.5 mm or more and 5 mm or less thick. The element main body 60 includesa gas-to-be-analyzed introduction port 61 formed in the fifth surface 60e, through which the measurement-object gas is introduced into theelement main body 60, and a reference gas introduction port 62 formed inthe sixth surface 60 f, through which a reference gas (in thisembodiment, air) used as a reference for detecting the particular gasconcentration is introduced into the element main body 60.

The detection unit 63 detects the specific gas concentration in themeasurement-object gas. The detection unit 63 includes a plurality ofelectrodes disposed in the front end-side part of the element main body60. In this embodiment, the detection unit 63 includes an outerelectrode 64 disposed on the first surface 60 a and an inner main pumpelectrode 65, an inner auxiliary pump electrode 66, a measurementelectrode 67, and a reference electrode 68 that are disposed inside theelement main body 60. The inner main pump electrode 65 and the innerauxiliary pump electrode 66 are disposed on the inner peripheral surfaceof a cavity formed inside the element main body 60 and have atunnel-like structure.

Since the principle on which the detection unit 63 detects the specificgas concentration in the measurement-object gas is publicly known,detailed description is omitted herein. The detection unit 63 detectsthe particular gas concentration, for example, in the following manner.The detection unit 63 draws oxygen included in the measurement-objectgas which is in the vicinity of the inner main pump electrode 65 to orfrom the outside (the element chamber 33) on the basis of the voltageapplied between the outer electrode 64 and the inner main pump electrode65. The detection unit 63 also draws oxygen included in themeasurement-object gas which is in the vicinity of the inner auxiliarypump electrode 66 to or from the outside (the element chamber 33) on thebasis of the voltage applied between the outer electrode 64 and theinner auxiliary pump electrode 66. This enables the measurement-objectgas to reach a space around the measurement electrode 67 after theoxygen concentration in the gas has been adjusted to be a predeterminedvalue. The measurement electrode 67 serves as a NOx-reducing catalystand reduces the particular gas (NOx) included in the measurement-objectgas. The detection unit 63 converts an electromotive force generatedbetween the measurement electrode 67 and the reference electrode 68 inaccordance with the oxygen concentration in the reduced gas or a currentthat flows between the measurement electrode 67 and the outer electrode64 on the basis of the electromotive force into an electrical signal.The electrical signal generated by the detection unit 63 indicates thevalue reflective of the particular gas concentration in themeasurement-object gas (the value from which the particular gasconcentration can be derived) and corresponds to the value detected bythe detection unit 63.

The heater 69 is an electric resistor disposed inside the element mainbody 60. Upon the heater 69 being fed with power from the outside, theheater 69 generates heat and heats the element main body 60. The heater69 is capable of heating the solid-electrolyte layers constituting theelement main body 60 and conserving the heat such that the temperatureis adjusted to be the temperature (e.g., 800° C.) at which thesolid-electrolyte layers become active.

The upper connector electrode 71 and the lower connector electrode 72are each disposed on the rear end-side part of any of the side surfacesof the element main body 60. The upper connector electrode 71 and thelower connector electrode 72 are electrodes that enable electricalconduction between the element main body 60 and the outside. The upperand lower connector electrodes 71 and 72 are not covered with the porouslayer 80 and exposed to the outside. In this embodiment, four upperconnector electrodes 71 a to 71 d, which serve as an upper connectorelectrode 71, are arranged in the left-to-right direction and disposedon the rear end-side part of the first surface 60 a, and four lowerconnector electrodes 72 a to 72 d, which serve as a lower connectorelectrode 72, are arranged in the left-to-right direction and disposedon the rear end-side part of the second surface 60 b (lower surface),which is opposite to the first surface 60 a (upper surface). Each of theconnector electrodes 71 a to 71 d and 72 a to 72 d is in electricalconduction with any of the electrodes 64 to 68 and the heater 69included in the detection unit 63. In this embodiment, the upperconnector electrode 71 a is in conduction with the measurement electrode67; the upper connector electrode 71 b is in conduction with the outerelectrode 64; the upper connector electrode 71 c is in conduction withthe inner auxiliary pump electrode 66; the upper connector electrode 71d is in conduction with the inner main pump electrode 65; the lowerconnector electrodes 72 a to 72 c are each in conduction with the heater69; and the lower connector electrode 72 d is in conduction with thereference electrode 68. The upper connector electrode 71 b and the outerelectrode 64 are in conduction with each other via an outer lead wire 75disposed on the first surface 60 a (see FIGS. 3 and 4 ). Each of theother connector electrodes is in conduction with a corresponding one ofthe electrodes and the heater 69 via a lead wire, through-hole, or thelike formed inside the element main body 60.

The porous layer 80 is a porous body that covers at least the frontend-side parts of the side surfaces of the element main body 60 on whichthe upper and lower connector electrodes 71 and 72 are disposed, thatis, the first and second surfaces 60 a and 60 b. In this embodiment, theporous layer 80 includes an inner porous layer 81 that covers the firstand second surfaces 60 a and 60 b and an outer porous layer 85 disposedon the outer surface of the inner porous layer 81.

The inner porous layer 81 includes a first inner porous layer 83 thatcovers the first surface 60 a and a second inner porous layer 84 thatcovers the second surface 60 b. The first inner porous layer 83 coversthe entirety of the region extending from the front end to the rear endof the first surface 60 a on which the upper connector electrodes 71 ato 71 d are disposed, except the regions in which a firstwater-penetration reduction portion 91 and the upper connector electrode71 are present (see FIGS. 2 to 4 ). The width of the first inner porouslayer 83 in the left-to-right direction is equal to the width of thefirst surface 60 a in the left-to-right direction. The first innerporous layer 83 covers the region that extends from the left end to theright end of the first surface 60 a. The first water-penetrationreduction portion 91 divides the first inner porous layer 83 into afront end-side portion 83 a located on the front end-side across thefirst water-penetration reduction portion 91 and a rear end-side portion83 b located on the rear end-side across the first water-penetrationreduction portion 91 in the longitudinal direction. The first innerporous layer 83 covers at least a part of the outer electrode 64 and atleast a part of the outer lead wire 75. In this embodiment, the firstinner porous layer 83 covers the entirety of the outer electrode 64 andthe entirety of the part of the outer lead wire 75 on which the firstwater-penetration reduction portion 91 is not present as illustrated inFIGS. 3 and 4 . The first inner porous layer 83 serves as, for example,a protection layer that protects the outer electrode 64 and the outerlead wire 75 from the components of the measurement-object gas, such assulfuric acid, and suppresses the corrosion and the like of the outerelectrode 64 and the outer lead wire 75.

The second inner porous layer 84 covers the entirety of the regionextending from the front end to the rear end of the second surface 60 bon which the lower connector electrodes 72 a to 72 d are disposed,except the regions in which a second water-penetration reduction portion94 and the lower connector electrode 72 are present (see FIGS. 2, 3, and5 ). The width of the second inner porous layer 84 in the left-to-rightdirection is equal to the width of the second surface 60 b in theleft-to-right direction. The second inner porous layer 84 covers theregion that extends from the left end to the right end of the secondsurface 60 b. The second water-penetration reduction portion 94 dividesthe second inner porous layer 84 into a front end-side portion 84 alocated on the front end-side across the second water-penetrationreduction portion 94 and a rear end-side portion 84 b located on therear end-side across the second water-penetration reduction portion 94in the longitudinal direction.

The outer porous layer 85 covers the first to fifth surfaces 60 a to 60e. The outer porous layer 85 covers the first surface 60 a and thesecond surface 60 b as a result of covering the inner porous layer 81.The length of the outer porous layer 85 in the front-to-rear directionis smaller than the length of the inner porous layer 81 in thefront-to-rear direction. The outer porous layer 85 covers only the frontend of the element main body 60 and a region of the element main body 60around the front end, unlike the inner porous layer 81. Thus, the outerporous layer 85 covers a part of the element main body 60 whichsurrounds the electrodes 64 to 68 included in the detection unit 63. Inother words, the outer porous layer 85 covers a part of the element mainbody 60 which is disposed inside the element chamber 33 and exposed tothe measurement-object gas. Thereby, the outer porous layer 85 servesas, for example, a protection layer that reduces the likelihood ofmoisture and the like included in the measurement-object gas adhering tothe element main body 60 and causing cracking of the element main body60.

The porous layer 80 is composed of, for example, a ceramic porous body,such as an alumina porous body, a zirconia porous body, a spinel porousbody, a cordierite porous body, a titania porous body, or a magnesiaporous body. In this embodiment, the porous layer 80 is composed of analumina porous body. The thicknesses of the first inner porous layer 83and the second inner porous layer 84 may be, for example, 5 μm or moreand 40 μm or less. The thickness of the outer porous layer 85 may be,for example, 40 μm or more and 800 μm or less. The porosity of theporous layer 80 is 10% or more. Although the porous layer 80 covers theouter electrode 64 and the gas-to-be-analyzed introduction port 61, themeasurement-object gas can pass through the porous layer 80 when theporosity of the porous layer 80 is 10% or more. The porosity of theinner porous layer 81 may be 10% or more and 50% or less. The porosityof the outer porous layer 85 may be 10% or more and 85% or less. Theouter porous layer 85 may have a higher porosity than the inner porouslayer 81.

The porosity of the inner porous layer 81 is determined by the followingmethod using an image (SEM image) obtained by inspecting the innerporous layer 81 with a scanning electron microscope (SEM). First, thesensor element 20 is cut in the thickness direction of the inner porouslayer 81 such that a cross section of the inner porous layer 81 can beinspected. The cross section is buried in a resin and ground in order toprepare an observation sample. An image of the observation cross sectionof the observation sample is taken with a SEM at a 1000 to 10000-foldmagnification in order to obtain an SEM image of the inner porous layer81. Subsequently, the image is subjected to image analysis. A thresholdvalue is determined on the basis of the brightness distribution includedin brightness data of pixels of the image by a discriminant analysismethod (Otsu's binarization). On the basis of the threshold value, thepixels of the image are binarized into an object portion and a poreportion. The areas of the object portions and the pore portions arecalculated. The ratio of the area of the pore portions to the total area(the total area of the object portions and the pore portions) iscalculated as a porosity (unit: %). The porosity of the outer porouslayer 85 and the porosities of the first and second dense layers 92 and95, which are described below, are also calculated by the same method asdescribed above.

The water-penetration reduction portion 90 reduces the capillarity ofwater through the element main body 60 in the longitudinal direction. Inthis embodiment, the water-penetration reduction portion 90 includes afirst water-penetration reduction portion 91 and a secondwater-penetration reduction portion 94. The first water-penetrationreduction portion 91 is disposed on the first surface 60 a, on which theupper connector electrode 71 and the first inner porous layer 83 aredisposed. As described above, the first water-penetration reductionportion 91 is disposed on the first surface 60 a so as to divide thefirst inner porous layer 83 into front and rear parts in thelongitudinal direction. The first water-penetration reduction portion 91is arranged closer to the front end of the element main body 60 than theupper connector electrode 71, that is, disposed forward of the upperconnector electrode 71. The first water-penetration reduction portion 91is disposed backward of the outer electrode 64. The firstwater-penetration reduction portion 91 is disposed backward of any ofthe electrodes 64 to 68 included in the detection unit 63, in additionto the outer electrode 64 (see FIG. 3 ). The first water-penetrationreduction portion 91 is arranged to overlap the insulator 44 b in thefront-to-rear direction (see FIG. 1 ). In other words, the region thatextends from the front end to the rear end of the firstwater-penetration reduction portion 91 is included in the region thatextends from the front end to the rear end of the insulator 44 b. Thefirst water-penetration reduction portion 91 blocks moisture that movesbackward inside the front end-side portion 83 a by capillarity frompassing through the first water-penetration reduction portion 91 andreduces the likelihood of the moisture reaching the upper connectorelectrode 71. The first water-penetration reduction portion 91 includesa first dense layer 92 and a first gap region 93. The first dense layer92 is a dense layer having a porosity of less than 10%. The width of thefirst dense layer 92 in the left-to-right direction is equal to thewidth of the first surface 60 a in the left-to-right direction. Thefirst dense layer 92 covers the first surface 60 a so as to extend fromthe left end to the right end of the first surface 60 a. The first denselayer 92 is adjacent to the rear end of the front end-side portion 83 a.The first dense layer 92 covers a part of the outer lead wire 75 asillustrated in FIG. 4 . The first gap region 93 is a region of the firstsurface 60 a in which the porous layer 80 and the first dense layer 92are not present. The first gap region 93 is a region between the rearend of the first dense layer 92 and the front end of the rear end-sideportion 83 b. The outer lead wire 75 is exposed to the outside at a partin which the first gap region 93 is present.

The second water-penetration reduction portion 94 is disposed on thesecond surface 60 b, on which the lower connector electrode 72 and thesecond inner porous layer 84 are disposed. As described above, thesecond water-penetration reduction portion 94 is disposed on the secondsurface 60 b so as to divide the second inner porous layer 84 into frontand rear parts in the longitudinal direction. The secondwater-penetration reduction portion 94 is arranged closer to the frontend of the element main body 60 than the lower connector electrode 72,that is, disposed forward of the lower connector electrode 72. Thesecond water-penetration reduction portion 94 is disposed backward ofthe outer electrode 64. The second water-penetration reduction portion94 is disposed backward of any of the electrodes 64 to 68 included inthe detection unit 63, in addition to the outer electrode 64 (see FIG. 3). The second water-penetration reduction portion 94 is arranged tooverlap the insulator 44 b in the front-to-rear direction (see FIG. 1 ).In other words, the region that extends from the front end to the rearend of the second water-penetration reduction portion 94 is included inthe region that extends from the front end to the rear end of theinsulator 44 b. The second water-penetration reduction portion 94 blocksmoisture that moves backward inside the front end-side portion 84 a bycapillarity from passing through the second water-penetration reductionportion 94 and reduces the likelihood of the moisture reaching the lowerconnector electrode 72. The second water-penetration reduction portion94 includes a second dense layer 95 and a second gap region 96. Thesecond dense layer 95 is a dense layer having a porosity of less than10%. The width of the second dense layer 95 in the left-to-rightdirection is equal to the width of the second surface 60 b in theleft-to-right direction. The second dense layer 95 covers the secondsurface 60 b so as to extend from the left end to the right end of thesecond surface 60 b. The second dense layer 95 is adjacent to the rearend of the front end-side portion 84 a. The second gap region 96 is aregion of the second surface 60 b in which the porous layer 80 and thesecond dense layer 95 are not present. The second gap region 96 is aregion between the rear end of the second dense layer 95 and the frontend of the rear end-side portion 84 b.

The length L of the first and second water-penetration reductionportions 91 and 94 in the longitudinal direction (see FIGS. 4 and 5 ) is0.5 mm or more. When the length L is 0.5 mm or more, the likelihood ofthe moisture passing through the first and second water-penetrationreduction portions 91 and 94 can be reduced to a sufficient degree. Thelength L may be 5 mm or more. The length L may be 25 mm or less or 20 mmor less. Although the first and second water-penetration reductionportions 91 and 94 have the same length L in this embodiment, they mayhave different lengths L.

The first and second dense layers 92 and 95 may be composed of any ofthe ceramics described above as examples of the material for the porouslayer 80, although the first and second dense layers 92 and 95 aredifferent from the porous layer 80 in that the porosity of the first andsecond dense layers 92 and 95 is less than 10%. In this embodiment, thefirst and second dense layers 92 and 95 are composed of an aluminaceramic. The thickness of the first and second dense layers 92 and 95may be, for example, 5 μm or more and 40 μm or less. The thickness ofthe first dense layer 92 is preferably equal to or larger than that ofthe first inner porous layer 83. Similarly, the thickness of the seconddense layer 95 is preferably equal to or larger than that of the secondinner porous layer 84. The porosity of the first and second dense layers92 and 95 is preferably 8% or less and is more preferably 5% or less.The smaller the porosity of the first and second dense layers 92 and 95,the higher the degree of reduction in the capillarity of water in thelongitudinal direction of the element main body 60 which is achieved bythe first and second dense layers 92 and 95.

The length Le of the first and second dense layers 92 and 95 in thelongitudinal direction (see FIGS. 4 and 5 ) is preferably 0.5 mm ormore. In such a case, the likelihood of the moisture passing through thefirst and second water-penetration reduction portions 91 and 94 in thelongitudinal direction can be reduced to a sufficient degree by usingonly the first and second dense layers 92 and 95, respectively. Thelength Le may be 5 mm or more. Although the first and second denselayers 92 and 95 have the same length Le in this embodiment, they mayhave different lengths Le.

The length Lg of the first gap region 93 and the second gap region 96 inthe longitudinal direction is preferably 1 mm or less. When the lengthLg is relatively small, the area of the parts of the side surfaces (inthis embodiment, the first and second surfaces 60 a and 60 b) of theelement main body 60 which are exposed to the outside, that is, theparts of the side surfaces which are not covered with any of the porouslayer 80, the first dense layer 92, and the second dense layer 95, canbe reduced. In particular, in this embodiment, the outer lead wire 75 isdisposed on the first surface 60 a, and the outer lead wire 75 isdisadvantageously exposed to the outside in the region in which thefirst gap region 93 is present. Setting the length Lg of the first gapregion 93 to be small reduces the area of the part of the outer leadwire 75 which is not covered with any of the porous layer 80 and thefirst dense layer 92.

The method for producing the gas sensor 10 is described below. First,the method for producing the sensor element 20 is described. In theproduction of the sensor element 20, first, a plurality of (in thisembodiment, six) unbaked ceramic green sheets that correspond to theelement main body 60 are prepared. In each of the green sheets, asneeded, notches, through-holes, grooves, and the like are formed bypunching or the like, and electrodes and wire patterns are formed byscreen printing. In addition, unbaked porous layers that are to beformed into the first inner porous layer 83 and the second inner porouslayer 84 after baking and unbaked dense layers that are to be formedinto the first and second dense layers 92 and 95 after baking are formedon the surfaces of the green sheets which correspond to the first andsecond surfaces 60 a and 60 b by screen printing. Subsequently, thegreen sheets are stacked on top of one another. The green sheets stackedon top of one another are an unbaked element main body that is to beformed into the element main body after baking and include unbakedporous layers and unbaked dense layers. The unbaked element main body isbaked to form the element main body 60 including the first inner porouslayer 83, the second inner porous layer 84, the first dense layer 92,and the second dense layer 95. Subsequently, the outer porous layer 85is formed by plasma spraying. Hereby, the sensor element 20 is prepared.For producing the porous layer 80, the first dense layer 92, and thesecond dense layer 95, gel casting, dipping, and the like can be used inaddition to screen printing and plasma spraying.

The gas sensor 10 that includes the sensor element 20 is produced.First, the sensor element 20 is inserted into the cylindrical body 41 soas to penetrate the cylindrical body 41 in the axial direction.Subsequently, the insulator 44 a, the compact 45 a, the insulator 44 b,the compact 45 b, the insulator 44 c, and the metal ring 46 are disposedin the gap between the inner peripheral surface of the cylindrical body41 and the sensor element 20 in this order. Then, the metal ring 46 ispressed in order to compress the compacts 45 a and 45 b. While thecompacts 45 a and 45 b are compressed, the diameter reduction portions43 c and 43 d are formed. Hereby, the element-sealing member 40 isproduced, and the gap between the inner peripheral surface of thecylindrical body 41 and the sensor element 20 is sealed. The protectivecover 30 is welded to the element-sealing member 40, and the nut 47 isattached to the element-sealing member 40. Hereby, the assembly 15 isproduced. Lead wires 55 attached to a rubber stopper 57 so as topenetrate the rubber stopper 57 and a connector 50 connected to the leadwires 55 are prepared. The connector 50 is connected to the rearend-side part of the sensor element 20. Subsequently, the externalcylinder 48 is fixed to the main fitting 42 by welding. Hereby, the gassensor 10 is produced.

In the case where the element main body 60, the first dense layer 92,and the second dense layer 95 are formed by baking the unbaked elementmain body and the unbaked dense layers as described above, the length Leof the first and second dense layers 92 and 95 is preferably 20 mm orless. Since the unbaked element main body and the unbaked dense layersmay have different shrinkage ratios during baking, if the length Le isexcessively large, the sensor element 20 may become warped duringbaking. When the length Le is 20 mm or less, the warpage of the sensorelement 20 during baking can be limited. The length Le of the first andsecond dense layers 92 and 95 is preferably 30% or less of the length ofthe element main body 60 in the longitudinal direction. When the abovecondition is satisfied, the warpage of the sensor element 20 duringbaking can also be limited.

An example of the application of the gas sensor 10 is described below.When the measurement-object gas flows inside the pipe 58 while the gassensor 10 is attached to the pipe 58 as illustrated in FIG. 1 , themeasurement-object gas passes through the inside of the protective cover30 and enters the element chamber 33. Consequently, the front end-sidepart of the sensor element 20 is exposed to the measurement-object gas.Upon the measurement-object gas passing through the porous layer 80,reaching the outer electrode 64, and reaching the inside of the sensorelement 20 through the gas-to-be-analyzed introduction port 61, thedetection unit 63 generates an electrical signal reflective of the NOxconcentration in the measurement-object gas, as described above. Theelectrical signal is drawn through the upper and lower connectorelectrodes 71 and 72. The NOx concentration can be determined on thebasis of the electrical signal.

The measurement-object gas may contain moisture, which may move insidethe porous layer 80 by capillarity. If the moisture reaches the upperand lower connector electrodes 71 and 72, which are exposed to theoutside, the water and the components dissolved in the water, such assulfuric acid, may cause rusting and corrosion of the upper and lowerconnector electrodes 71 and 72 and a short circuit between some of theupper and lower connector electrodes 71 and 72 which are adjacent to oneanother. However, in this embodiment, even when the moisture containedin the measurement-object gas moves inside the porous layer 80 (inparticular, inside the first inner porous layer 83 and the second innerporous layer 84) toward the rear end-side part of the element main body60 by capillarity, the moisture reaches the first water-penetrationreduction portion 91 or the second water-penetration reduction portion94 before reaching the upper and lower connector electrodes 71 and 72.The first water-penetration reduction portion 91 includes the firstdense layer 92 having a porosity of less than 10% and the first gapregion 93 that is a space in which the porous layer is absent, and bothof them reduce the capillarity of water in the longitudinal direction ofthe element main body 60. In addition, since the length L of the firstwater-penetration reduction portion 91 in the longitudinal direction is0.5 mm or more, the likelihood of moisture passing through the firstwater-penetration reduction portion 91 can be reduced to a sufficientdegree. By the above mechanisms, the first water-penetration reductionportion 91 reduces the likelihood of the moisture passing through thefirst water-penetration reduction portion 91 from the front end-sideportion 83 a-side and reaching the upper connector electrode 71 (theupper connector electrodes 71 a to 71 d). Therefore, in the sensorelement 20, the above-described trouble caused by the water adhering tothe upper connector electrode 71 may be reduced. In the similar manneras described above, the second water-penetration reduction portion 94,which includes the second dense layer 95 and the second gap region 96,reduces the likelihood of the moisture passing through the secondwater-penetration reduction portion 94 from the front end-side portion84 a-side and reaching the lower connector electrode 72 (the lowerconnector electrodes 72 a to 72 d). Therefore, in the sensor element 20,the above-described trouble caused by the water adhering to the lowerconnector electrode 72 may be reduced.

The correspondences between the elements constituting this embodimentand the elements constituting the present invention are explicitlydescribed below: the element main body 60 in this embodiment correspondsto the element main body in the present invention; the detection unit 63corresponds to the detection unit; the connector electrodes 71 a to 71 dand 72 a to 72 d correspond to the connector electrodes; the firstsurface 60 a and the second surface 60 b correspond to the side surfaceon which the connector electrodes are disposed; the porous layer 80corresponds to the porous layer; the first and second water-penetrationreduction portions 91 and 94 each correspond to the water-penetrationreduction portion; the outer lead wire 75 corresponds to the outer leadportion; the outer electrode 64 corresponds to the outer electrode; thefirst surface 60 a corresponds to the first side surface; and the secondsurface 60 b corresponds to the second side surface.

Since the sensor element 20 according to this embodiment described abovein detail includes the first water-penetration reduction portion 91disposed on any of the one or more side surfaces (in this embodiment,the first surface 60 a) of the element main body 60, the likelihood ofthe moisture passing through the porous layer 80 (in this embodiment, inparticular, the first inner porous layer 83) and reaching the upperconnector electrodes 71 a to 71 d can be reduced. In the same manner asabove, since the sensor element 20 includes the second water-penetrationreduction portion 94 disposed on any of the one or more side surfaces(in this embodiment, the second surface 60 b) of the element main body60, the likelihood of the moisture passing through the porous layer 80(in this embodiment, in particular, the second inner porous layer 84)and reaching the lower connector electrodes 72 a to 72 d can also bereduced.

Since the length Le of the first and second dense layers 92 and 95 is0.5 mm or more, the likelihood of the moisture passing through thewater-penetration reduction portion 90 in the longitudinal direction canbe reduced to a sufficient degree by using only the first and seconddense layers 92 and 95 of the first and second water-penetrationreduction portions 91 and 94, respectively. Since the length Le of thefirst and second dense layers 92 and 95 is 20 mm or less, the warpage ofthe sensor element 20 caused due to the difference in shrinkage ratioduring baking between the unbaked element main body and the unbakeddense layers can be reduced. Since the length Le of the first and seconddense layers 92 and 95 is 30% or less of the length of the element mainbody 60 in the longitudinal direction, the warpage of the sensor element20 can be further reduced.

Since the length Lg of the first and second gap regions 93 and 96 is 1mm or less, that is, relatively small, the area of parts of the sidesurfaces (in this embodiment, the first and second surfaces 60 a and 60b) of the element main body 60 which are exposed to the outside (theparts that are not covered with any of the porous layer 80, the firstdense layers 92, and the second dense layer 95) can be reduced.

The sensor element 20 includes an outer lead wire 75 that is disposed onthe side surface (in this embodiment, the first surface 60 a) on whichthe upper connector electrode 71 is disposed and that provideselectrical conduction between any of the electrodes (in this embodiment,the outer electrode 64) included in the detection unit 63 and the upperconnector electrode 71 b. The porous layer 80 (in particular, the firstinner porous layer 83) covers at least a part of the outer lead wire 75.Consequently, at least a part of the outer lead wire 75 can be protectedby the porous layer 80. In the case where the outer lead wire 75 isprotected by the porous layer 80, the porous layer (in this embodiment,the first inner porous layer 83) is likely to be formed at a positionclose to the upper connector electrode 71 b. In such a case, it ismeaningful to reduce the likelihood of the moisture passing through thefirst inner porous layer 83 and reaching the upper connector electrode71 b by using the first water-penetration reduction portion 91.

It is to be understand that the present invention is not limited to theabove-described embodiment at all, but intended to include a variety offorms within the technical scope of the present invention.

For example, although the first water-penetration reduction portion 91includes the first dense layer 92 and the first gap region 93 in theabove-described embodiment, the first water-penetration reductionportion 91 includes at least the first dense layer 92. That is, thefirst water-penetration reduction portion 91 does not necessarilyinclude the first gap region 93. In other words, the length Lg in thefirst water-penetration reduction portion 91 may be 0 mm. FIG. 6 is atop view of such a sensor element 20. When the first water-penetrationreduction portion 91 does not include the first gap region 93, the areaof a part of the first surface 60 a which is exposed to the outside (theportion that is not covered with any of the porous layer 80 and thefirst dense layer 92) can be further reduced. The same applies to thesecond water-penetration reduction portion 94.

Although the first water-penetration reduction portion 91 divides thefirst inner porous layer 83 into the front end-side portion 83 a and therear end-side portion 83 b in the longitudinal direction in theabove-described embodiment, the present invention is not limited tothis. The first water-penetration reduction portion 91 may be arrangedcloser to the rear end than the porous layer 80. For example, in theabove-described embodiment, the first inner porous layer 83 does notnecessarily include the rear end-side portion 83 b. In such a case, theregion in which the rear end-side portion 83 b is disposed in FIG. 4 isconsidered a part of the first gap region 93. Similarly to the firstwater-penetration reduction portion 91, the second water-penetrationreduction portion 94 may be arranged closer to the rear end than theporous layer 80 instead of dividing the second inner porous layer 84into two parts.

Although the first dense layer 92 is disposed forward of the first gapregion 93 so as to be adjacent to the first gap region 93 in theabove-described embodiment, the first dense layer 92 may be disposedbackward of the first gap region 93 so as to be adjacent to the firstgap region 93. The same applies to the second water-penetrationreduction portion 94.

Although the first and second water-penetration reduction portions 91and 94 are arranged to overlap the insulator 44 b in the front-to-reardirection in the above-described embodiment, the present invention isnot limited to this. For example, the first and second water-penetrationreduction portions 91 and 94 may be arranged to overlap the insulator 44a or the insulator 44 c in the front-to-rear direction or may bedisposed backward of the metal ring 46. The first and secondwater-penetration reduction portions 91 and 94 are preferably disposedso as not to be exposed to the inside of the element chamber 33.

In the above-described embodiment, the sensor element 20 does notnecessarily include the second inner porous layer 84 and the secondsurface 60 b is not necessarily covered with the porous layer 80. Insuch a case, the sensor element 20 does not necessarily include thesecond water-penetration reduction portion 94. The water-penetrationreduction portion may be disposed on at least one of the side surfacesof the element main body (in the above-described embodiment, the firstto fourth surfaces 60 a to 60 d) on which the connector electrodes andthe porous protection layer are disposed (in the above-describedembodiment, the first and second surfaces 60 a and 60 b). This reducesthe likelihood of the moisture reaching the connector electrodes atleast on the side surface on which the water-penetration reductionportion is disposed.

Although the first inner porous layer 83 covers the region that extendsfrom the front to rear ends of the first surface 60 a except the regionin which the first water-penetration reduction portion 91 and the upperconnector electrode 71 are present in the above-described embodiment,the present invention is not limited to this. For example, the firstinner porous layer 83 may cover a region that extends from the front endof the first surface 60 a to the front end-side ends of the upperconnector electrodes 71 a to 71 d except the region in which the firstwater-penetration reduction portion 91 is present. Alternatively, thefirst inner porous layer 83 may cover at least a region that extendsfrom the front end of the first surface 60 a to the rear of the firstwater-penetration reduction portion 91 except the region in which thefirst water-penetration reduction portion 91 is present. The sameapplies to the second inner porous layer 84.

Although the element main body 60 has a rectangular cuboid shape in theabove-described embodiment, the present invention is not limited tothis. For example, the element main body 60 may have a hollowcylindrical shape or a solid cylindrical shape. In such a case, theelement main body 60 has one side surface.

EXAMPLES

Example cases where a specific sensor element was prepared are describedbelow as Examples. Experimental examples 5 to 7, 9, 10, 12 to 14, 16 to18, 20, 21, 23, 24, 26, 27, 29, and 30 correspond to Examples of thepresent invention, while Experimental examples 1 to 4, 8, 11, 15, 19,22, 25, 28, and 31 correspond to Comparative examples. Note that thepresent invention is not limited by Examples below.

Experimental Example 1

In Experimental example 1, a sensor element that was the same as thesensor element 20 illustrated in FIGS. 2 to 5 , except that the sensorelement did not include the first water-penetration reduction portion91, the second water-penetration reduction portion 94, and the outerporous layer 85, was prepared. That is, in Experimental example 1, thefirst and second inner porous layers 83 and 84 were arranged to coverthe entirety of the first and second surfaces 60 a and 60 b except theregion in which the upper and lower connector electrodes 71 and 72 weredisposed. The sensor element 20 of the Experimental example 1 wasprepared in the following manner. First, zirconia particles containing 4mol % yttria serving as a stabilizer were mixed with an organic binderand an organic solvent. The resulting mixture was formed into sixceramic green sheets by tape casting. Patterns of electrodes and thelike were printed in each of the green sheets. In addition, unbakedporous layers that were to be formed into the first inner porous layer83 and the second inner porous layer 84 after baking were formed byscreen printing. The unbaked porous layers were composed of a slurryprepared by mixing a raw-material powder (an alumina powder), a bindersolution (polyvinyl acetal and butyl carbitol), a solvent (acetone), anda pore-forming material with one another. Subsequently, the six greensheets were stacked on top of one another and baked in order to preparethe element main body 60 including the first and second inner porouslayers 83 and 84. Hereby, the sensor element 20 of Experimental example1 was prepared. The dimensions of the element main body 60 were 67.5 mmlong, 4.25 mm wide, and 1.45 mm thick. The first and second inner porouslayers 83 and 84 had a thickness of 20 μm and a porosity of 30%.

Experimental Examples 2 to 31

In Experimental examples 2 to 31, a sensor element that was the same asthat prepared in Experimental example 1, except that the sensor elementincluded the first and second water-penetration reduction portions 91and 94, was prepared. In Experimental examples 2 to 31, the length Leand the porosity of the first and second dense layers 92 and 95, thelength Lg of the first and second gap regions 93 and 96, and the lengthL of the first and second water-penetration reduction portions 91 and 94were changed as described in Table 1. The unbaked dense layers that wereto be formed into the first and second dense layers 92 and 95 afterbaking were formed using a slurry that was the same as that used forforming the unbaked porous layers in Experimental example 1, except thatthe amount of the pore-forming material added to the slurry was reduced.In Experimental examples 2 to 31, the porosity of the first and seconddense layers 92 and 95 was changed by adjusting the amount of thepore-forming material added to the slurry. In Experimental examples 2 to31 except Experimental example 5, the first water-penetration reductionportion 91 did not include the first gap region 93 and the secondwater-penetration reduction portion 94 did not include the second gapregion 96 as illustrated in FIG. 6 . In Experimental examples 2 to 31,the first and second dense layers 92 and 95 had a thickness of 20 μm. InExperimental examples 2 to 31, the front ends of the first and secondwater-penetration reduction portions 91 and 94 were located at aposition 26 mm from the front end of the element main body 60.

[Liquid Penetration Test]

Each of the sensor elements 20 prepared in Experimental examples 1 to 31was tested in order to determine the amount of liquid that penetratedthe rear end-side part of the element main body 60 by capillarity whenthe front end-side part of the element main body 60 was immersed in theliquid. First, while the sensor element 20 was held such that thelongitudinal direction of the sensor element 20 was parallel to thevertical direction, a part of the sensor element 20 which extended fromthe front end (fifth surface 60 e) of the element main body 60 to aposition (hereinafter, “immersion position”) 20 mm from the front endtoward the rear end was immersed into a red-check solution. While thesensor element was immersed in the red-check solution, the sensorelement was left to stand for 20 hours. Subsequently, the distance thered-check solution penetrated from the immersion position toward therear end was measured visually as a penetration distance. Thepenetration distance indicates the distance the red-check solution movedfrom the immersion position toward the rear end of the element main body60 inside the first and second inner porous layers 83 and 84 bycapillarity. An evaluation grade of Excellent (A) was given when thepenetration distance measured after a lapse of 20 hours was less than 10mm. An evaluation grade of Good (B) was given when the penetrationdistance measured after a lapse of 20 hours was 10 mm or more and lessthan 20 mm. An evaluation grade of Failure (F) was given when thepenetration distance measured after a lapse of 20 hours was 20 mm ormore. The red-check solution used was “R-3B(NT) PLUS” produced by EishinKagaku Co., Ltd. The red-check solution included 40 to 60 wt %hydrocarbon oil, 10 to 20 wt % plastic solvent, 1 to 20 wt % glycolether, 12 to 50 wt % non-ionic surfactant, and 1 to 5 wt % oil-solubleazo red dye. The red-check solution had a density of 0.86 g/cm³ at 20°C., which was lower than the density of water.

[Evaluation of Warpage of Sensor Element]

The amount of warpage of each of the sensor elements 20 prepared inExperimental examples 1 to 31 in the thickness direction (top-to-bottomdirection) of the sensor element 20 was measured with a laserdisplacement sensor “LK-010” produced by Keyence Corporation. InExperimental example 1, 100 sensor elements 20 were prepared, and theamount of warpage of each of the 100 sensor elements was measured. Whenthe amount of warpage of a sensor element was 200 μm or more, the sensorelement was considered warped. The number of warped sensor elements ofthe 100 sensor elements was counted, and the warpage occurrence rate wascalculated. In Experimental examples 2 to 31, the warpage occurrencerate was calculated as in Experimental example 1. An evaluation ofExcellent (A) was given when the warpage occurrence rate was less than1% (any of the 100 sensor elements did not become warped). An evaluationof Good (B) was given when the warpage occurrence rate was 1% or moreand less than 20%. An evaluation of Failure (F) was given when thewarpage occurrence rate was 20% or more.

Table 1 summarizes the length Le, the porosity of the first and seconddense layers 92 and 95, the length Lg, the length L, the results ofevaluation of the liquid penetration test, and the results of evaluationof warpage of the sensor element 20 in each of Experimental examples 1to 31. FIG. 7 is a graph illustrating changes in penetration distancewith time which were measured in the liquid penetration tests conductedin Experimental examples 1 and 16.

TABLE 1 Length L of water- Length Le of Porosity of Length Lg ofpenetration reduction Liquid Warpage of dense layer dense layer gapregion portion penetration sensor [mm] [%] [mm] (= Le + Lg)[mm] testelement Experimental example 1 0 — 0 0 F A Experimental example 2 0.3 00 0.3 F A Experimental example 3 5 F A Experimental example 4 10 F AExperimental example 5 0 0.2 0.5 A A Experimental example 6 0.5 0 0 0.5A A Experimental example 7 5 B A Experimental example 8 10 F AExperimental example 9 1 0 0 1 A A Experimental example 10 5 B AExperimental example 11 10 F A Experimental example 12 3 0 0 3 A AExperimental example 13 5 A A Experimental example 14 8 B A Experimentalexample 15 10 F A Experimental example 16 5 0 0 5 A A Experimentalexample 17 5 A A Experimental example 18 8 B A Experimental example 1910 F A Experimental example 20 10 0 0 10 A A Experimental example 21 5 AA Experimental example 22 10 F A Experimental example 23 15 0 0 15 A AExperimental example 24 5 A A Experimental example 25 10 F AExperimental example 26 20 0 0 20 A B Experimental example 27 5 A BExperimental example 28 10 F A Experimental example 29 27 0 0 27 A FExperimental example 30 5 A F Experimental example 31 10 F F

The results illustrated in FIG. 7 show that, in Experimental example 1where the first and second water-penetration reduction portions 91 and94 were absent, the penetration distance was increased with time. Thisconfirms that the red-check solution moved inside the first and secondinner porous layers 83 and 84 toward the rear of the sensor element 20by capillarity. In contrast, in Experimental example 16 where the firstand second water-penetration reduction portions 91 and 94 were present,the red-check solution reached a penetration position of 6 mm (=theposition 26 mm from the front end of the element main body 60) that wasthe position of the front ends of the first and second water-penetrationreduction portions 91 and 94. This confirms that, in Experimentalexample 16, the first and second water-penetration reduction portions 91and 94 blocked the red-check solution from moving backward bycapillarity.

The results described in Table 1 show that, in Experimental examples 5to 7, 9, 10, 12 to 14, 16 to 18, 20, 21, 23, 24, 26, 27, 29, and 30where the length L was 0.5 mm or more and the porosity of the first andsecond dense layers 92 and 95 was less than 10%, the results of theliquid penetration test were evaluated as Excellent or Good. Incontrast, in Experimental examples 1 to 4 where the length L was lessthan 0.5 mm and Experimental examples 8, 11, 15, 19, 22, 25, 28, and 31where the porosity of the first and second dense layers 92 and 95 was10%, the results of the liquid penetration test were evaluated asFailure. This confirms that, when length L of the first and secondwater-penetration reduction portions 91 and 94 is 0.5 mm or more and theporosity of the first and second dense layers 92 and 95 is less than10%, the movement of the moisture by capillarity can be reduced by thefirst and second water-penetration reduction portions 91 and 94 to asufficient degree. The results obtained in Experimental example 5 showthat, even in the case where the length Le of the first and second denselayers 92 and 95 was less than 0.5 mm, the movement of the moisture bycapillarity was suppressed to a sufficient degree when the length L ofthe first and second water-penetration reduction portions 91 and 94 was0.5 mm or more. A comparison between Experimental examples 6 and 7 and acomparison among Experimental examples 12 to 14 confirm that, the lowerthe porosity of the first and second dense layers 92 and 95, the greaterthe suppression of the movement of the moisture by capillarity. It wasalso confirmed that, the larger the length Le of the first and seconddense layers 92 and 95, the lower the likelihood of the evaluation gradeof the liquid penetration test being lowered from A to B even when theporosity of the first and second dense layers 92 and 95 is high.

The results described in Table 1 show that, in Experimental examples 29to 31 where the length Le of the first and second dense layers 92 and 95was more than 20 mm (=the length Le was 40% of the length (67.5 mm) ofthe element main body 60), the sensor element 20 was likely to becomewarped, and an evaluation grade of Failure was given. In Experimentalexamples 1 to 28 where the length Le was 20 mm or less (the length Lewas 30% or less of the length (67.5 mm) of the element main body 60), anevaluation grade of “A” or “B” was given. This confirms that the warpageof the sensor element 20 was reduced. It was also confirmed that, thesmaller the length Le, the lower the likelihood of the sensor element 20becoming warped.

What is claimed is:
 1. A sensor element comprising: a long-lengthelement main body including front and rear ends and one or more sidesurfaces, the front and rear ends being ends of the element main body ina longitudinal direction of the element main body, the one or more sidesurfaces being surfaces extending in the longitudinal direction; adetection unit including a plurality of electrodes disposed in the frontend-side part of the element main body, the detecting unit detecting aspecific gas concentration in a measurement-object gas; one or moreconnector electrodes disposed on the rear end-side part of any of theone or more side surfaces, the one or more connector electrodes used forelectrical connection to outside of the sensor element; a porous layerthat covers at least the front end-side part of the one or more sidesurface on which the one or more connector electrodes are disposed, theporous layer having a porosity of 10% or more; and a water-penetrationreduction portion disposed on the one or more side surfaces so as todivide the porous layer in the longitudinal direction or to be locatedcloser to the rear end than the porous layer, the water-penetrationreduction portion being located closer to the front end than the one ormore connector electrodes, a length (L) of the water-penetrationreduction portion in the longitudinal direction being 0.5 mm or more,the water-penetration reduction portion including a dense layer coveringthe one or more side surfaces and having a porosity of less than 10% anda gap region in which the porous layer is absent, the gap region beingarranged adjacent to the dense layer and at least the dense layerreducing capillarity of water in the longitudinal direction, and thedense layer and a portion of the porous layer located closer to thefront end than the dense layer are adjacent to each other, or the denselayer and the portion of the porous layer are separated from each otherwith the gap region disposed in between.
 2. The sensor element accordingto claim 1, wherein a length (Le) of the dense layer in the longitudinaldirection is 0.5 mm or more.
 3. The sensor element according to claim 1,wherein a length (Le) of the dense layer in the longitudinal directionis 20 mm or less.
 4. The sensor element according to claim 1, wherein alength (Lg) of the gap region in the longitudinal direction is 1 mm orless.
 5. The sensor element according to claim 1, further comprising anouter lead portion disposed on the side surface on which the one or moreconnector electrodes are disposed, the outer lead portion providingelectrical conduction between any of the plurality of electrodes and oneof the one or more connector electrodes, wherein the porous layer coversat least a part of the outer lead portion.
 6. The sensor elementaccording to claim 1, wherein the porous layer covers at least a regionof the side surface on which the one or more connector electrodes aredisposed, the region extending from the front end of the side surface tothe front end-side edges of the one or more connector electrodes, theregion excluding a region in which the water-penetration reductionportion is present, and the water-penetration reduction portion isdisposed on the side surface so as to divide the porous layer in thelongitudinal direction.
 7. The sensor element according to claim 1,wherein the element main body has a rectangular cuboid shape with fourside surfaces that are the one or more side surfaces extending in thelongitudinal direction, wherein the one or more connector electrodes aredisposed on each of first and second side surfaces of the four sidesurfaces, the first and second side surfaces facing each other, theporous layer covers each of the first and second side surfaces, and thewater-penetration reduction portion is disposed on each of the first andsecond side surfaces.
 8. A gas sensor comprising the sensor elementaccording to claim
 1. 9. A sensor element comprising: a long-lengthelement main body including front and rear ends and one or more sidesurfaces, the front and rear ends being ends of the element main body ina longitudinal direction of the element main body, the one or more sidesurfaces being surfaces extending in the longitudinal direction; adetection unit including a plurality of electrodes disposed in the frontend-side part of the element main body, the detecting unit detecting aspecific gas concentration in a measurement-object gas; one or moreconnector electrodes disposed on the rear end-side part of any of theone or more side surfaces, the one or more connector electrodes used forelectrical connection to outside of the sensor element; a porous layerthat covers at least the front end-side part of the one or more sidesurfaces on which the one or more connector electrodes are disposed, theporous layer having a porosity of 10% or more; and a water-penetrationreduction portion disposed on the one or more side surfaces so as todivide the porous layer in the longitudinal direction or to be locatedcloser to the rear end than the porous layer, the water-penetrationreduction portion being located closer to the front end than the one ormore connector electrodes, a length (L) of the water-penetrationreduction portion in the longitudinal direction being 0.5 mm or more,the water-penetration reduction portion including a dense layer coveringthe one or more side surfaces and having a porosity of less than 10%, atleast the dense layer reducing capillarity of water in the longitudinaldirection and the dense layer and a portion of the porous layer locatedcloser to the front end than the dense layer are adjacent to each other.10. A sensor element according to claim 9, wherein a length (Le) of thedense layer in the longitudinal direction is 0.5 mm or more.
 11. Asensor element according to claim 9, wherein a length (Le) of the denselayer in the longitudinal direction is 20 mm or less.
 12. A sensorelement according to claim 9, wherein the sensor element furtherincludes an outer lead portion disposed on the side surface on which theone or more connector electrodes are disposed, the outer lead portionproviding electrical conduction between any of the plurality ofelectrodes and one of the one or more connector electrodes, and whereinthe porous layer covers at least a part of the outer lead portion.
 13. Asensor element according to claim 9, wherein the porous layer covers atleast a region of the side surface on which the one or more connectorelectrodes are disposed, the region extending from the front end of theside surface to the front end-side edges of the one or more connectorelectrodes, the region excluding a region in which the water-penetrationreduction portion is present, and wherein the water-penetrationreduction portion is disposed on the side surface so as to divide theporous layer in the longitudinal direction.
 14. A sensor elementaccording to claim 9, wherein the element main body has a rectangularcuboid shape with four side surfaces that are the one or more sidesurfaces extending in the longitudinal direction, wherein the one ormore connector electrodes are disposed on each of first and second sidesurfaces of the four side surfaces, the first and second side surfacesfacing each other, wherein the porous layer covers the first and secondside surfaces, and wherein the water-penetration reduction portion isdisposed on each of the first and second side surfaces.
 15. A gas sensorcomprising the sensor element according to claim 9.